1. Field of the Invention:
The present invention relates to a high resolution integrated-photocircuit interferometer capable of finding the direction of phase variation of the measured light, or the direction in which the optical path of the measured light increases or decreases, on the basis of an interference signal which varies in response to the change in the optical path length of the measured light.
2. Description of the Prior Art:
A recursive optical system type integrated-photocircuit interferometer is disclosed in Japanese Utility Model Publication No. 15522/1981, which is illustrated in FIGS. 13 and 14.
In FIGS. 13 and 14, 1 denotes a thin-walled substrate in which a two-dimensional wave-guiding paths are formed. The thin-walled substrate 1 consists of three thin layers 2, 3 and 4. Of these layers, at least the intermediate one 2 is capable of transmitting light. The thin layer 2 has a refractive index which is greater than those of the layers 3 and 4 on both sides. Coherent light P emitted from a light source 5 enters the substrate 1. The coherent light P is reflected by both of the interfaces between the layer 2 and the contiguous layers 3 and 4 and is propagated within the intermediate layer 2. The substrate 1 has a collimator lens system 6. The coherent light P entering the layer 2 is converted into a parallel beam of rays. The parallel beam is split into a reference beam P.sub.1 and a measurement beam P.sub.2 by a half mirror system 7. The reference beam P.sub.1 is reflected by a reference mirror 10 formed in the substrate 1 and is thus returned back to the half mirror system 7. The measurement beam P.sub.2 is reflected by the object or mirror 9 for measurement and is thus returned back to the half mirror system 7. The thus returned measurement beam P.sub.2 and reference beam P.sub.1 are united by the half mirror system 7 to interfere with each other, and the resultant interference beam is guided to a measuring lens 11. The interference beam is projected out of the layer through a prism 8.
The interference beam emitted from the prism 8 is dark when the difference between the optical paths of the measurement beam P.sub.2 and of the reference beam P.sub.1 is an odd multiple of .lambda./2, .lambda. being the wavelength of the coherent light P, while it is bright when the difference is an even multiple of .lambda./2. Therefore, a movement of the mirror 9 for measurement in the direction of arrow G will produce an interference signal depending on the interference beam, which signal changes alternately from dark B to bright A and then from bright A to dark B, as shown in FIG. 15, each time the amount of movement increases by .lambda./2. Therefore, it is possible to derive the amount of movement of the mirror 9 for measurement by counting the bright and dark portions A and B, and thus it is possible to determine the length of the observed object by counting the amount of the movement from the origin.
Such prior art integrated-photocircuit interferometer, however, has a disadvantage in that a movement of the mirror for measurement in the opposite direction H will cause a similar alternate pattern of bright and dark levels A and B in the signal, and hence it is impossible to determine the direction of change in the phase of the measurement beam. Besides, the integrated photocircuit interferometer has another disadvantage in that it is difficult with this apparatus to have a resolution finer than .lambda./2. Further, in this type of interferometer, part of the measurement beam P.sub.2 reflected by the mirror 9 will be reflected by the half mirror 7 and will return back to the light source 5. Similarly, part of the reference beam P.sub.1 will return back to the light source 5 after passing through the half mirror 7. This returning light will affect the output of the light source 5, interfering with the fine measurement.